1. Field of the Invention
The embodiments of the invention generally relate to semiconductor structures and methods, and more particularly to a fin-type field effect transistor (FinFET) formation method and resulting structure where impurity concentrations within the fins are asymmetric and mirror one another in adjacent pairs of fins.
2. Description of the Related Art
FinFETs are field-effect transistors (FET)s where the body of the FET is a block or fin of single-crystal semiconductor material and gates are formed on the fin. For a complete discussion of FinFETs, see U.S. Pat. No. 7,323,374 the complete disclosure of which is hereby incorporated by reference.
Angled implants of impurities (dopants) are especially useful in transistors that utilize fin structures because of the rectangular shape of the fin. Fins have sidewalls that are many times (at least to 2×) greater in height (above the insulator substrate) than the fin is wide. Therefore, it is more effective to apply implants to the relatively larger sidewalls than it is to implant impurities through the relatively smaller top of the fin (in a process which is sometimes referred to as “top-down” implanting).
Top-down implanting has limitations because, in order to attain homogeneous doping throughout the fin, top down implanting requires very high energy implants and high thermal cycles. Other limitations of top-down implanting relate to the amorphization limit and large lateral gradient. Therefore, angling the implants is a more effective process for implanting impurities into fin shaped structures.
However, one problem that occurs when implanting impurities into fin-type transistors is that, as the size of transistors is reduced, shadowing can occur because of various structures, such as masks, that can be adjacent to certain fins. This shadowing can prevent the formation of homogeneous doping.
In one example, the mask utilized for well and source/drain doping can cause some, but not all, of the fins within a multi-fin transistor to be shadowed preventing all the fins from receiving the full impurity implant. For example, fins that are closest to the mask structure may be shadowed, while fins that are more interior to the structure (and are further away from the mask) may not be shadowed. This causes inconsistent doping within the fins of the transistors when angled implants are utilized. Such inconsistent doping can cause the transistors to be unbalanced, and shadowing can occur intermittently, causing the performance of the transistors also to be inconsistent.